As a result of the increased participation of Americans in athletic activities and fitness programs, a dramatic increase in the number and type of activity-related injuries has occurred. The long-term objective of this proposal is to investigate the mechanism of muscle injury which occurs as a result of eccentric contractions, i.e. during lengthening of muscle while it is activation. Specifically, we propose experiments which determine whether muscle damage is cue to the amount of muscle lengthening (strain), the rate of muscle lengthening (strain rate), or the total muscle tension (stress). In this study, the tibialis anterior and soleus muscles will serve as model skeletal muscles and will be subjected to eccentric contraction, isometric contraction, and passive stretch at various stresses, strains and strain rates in order to identify which of these mechanical variables is most related to the level of injury observed. Injury will be determined by measuring the contractile properties of the muscle following one of the three treatments. In addition, muscle morphological properties will be examined and the light microscopic and electron microscopic level. The quantity of damage will be determined using stereological methods. In the final two parts of this proposal, three treatments which are thought to prevent muscle injury will be investigated, namely, "stretching "warming up," and "getting into shape." Next, methods which are thought to ameliorate recovery from muscle injury will be investigated, namely, "stretching out," nonsteroidal anti- inflammatory drugs, and isometric stimulation. As a result of these studies, our improved understanding of muscle injury will permit prescription of exercises and movements which could decrease injury and facilitate recovery following injury.